1). Field of the Invention
This invention relates generally to a method of making a transistor and more specifically to a method according to which side wall spacers of the transistor are made.
2). Discussion of Related Art
Electronic circuits are often manufactured in and on semiconductor wafers. Such an electronic circuit often includes millions of tiny transistors. Such a transistor usually includes a gate dielectric layer formed on the semiconductor material of the wafer, followed by a gate having a width in the region of 0.15 microns. Ions are implanted next to the gate to form lightly doped regions. Spacers are then formed adjacent side walls of the gate and more ions are implanted adjacent the spacers. The spacers shield an area of the semiconductor material near the gate from the higher concentration of ions due to the second implantation step.
The formation of the spacers usually involves the deposition of a silicon nitride layer over and next to the gate, followed by an anisotropic etch which removes upper surfaces of the silicon nitride layer until the gate is exposed and surfaces of the semiconductor material outside the spacers are exposed for the second ion implantation step. The silicon nitride layer is deposited by introducing relatively high concentrations of SiH4 and NH3 gases into a chamber, which react with one another to form silicon nitride which then deposits out. Due to the high concentrations of these gases and other factors such as temperature, pressure, and flow rate, hydrogen is usually trapped within the silicon nitride layer. The hydrogen may diffuse into the gate and the semiconductor material thus affecting functioning of the transistor.
The invention provides a method of making a transistor. A gate dielectric layer is formed on a semiconductor substrate. A gate is formed on the dielectric layer, the gate having an exposed upper surface and exposed side surfaces. A first silicon nitride layer having a first thickness is deposited over the gate, for example over an oxide layer on the gate, at a first deposition rate. A second layer having a second thickness is deposited over the first silicon nitride layer at a second deposition rate, the second thickness being more than the first thickness and the second deposition rate being more than the first deposition rate. The second layer is then etched to leave spacers next to the side surfaces while exposing the upper surface of the gate and areas of the substrate outside the spacers. The first silicon nitride layer then has a lower hydrogen concentration than the second layer.